Immunogenicity in the past year we have succeeded in identifying all the major B cell epitopes recognized by the human immune system and have made a new immunotoxin, HA22-LR-010, that is highly active and whose reactivity with human anti-sera is greatly reduced. We are continuing to further decrease reactivity by making further mutations in the new immunotoxin. We have also initiated a program to identify and remove human specific T cell epitopes. We have now identified all the major T cell epitopes and using alanine scanning mutagenisis to identify amino acids essential for T cell activation. We are attempting to make a new immunotoxin with mutations that will eliminate both B and T cell epitopes. In collaboration with D. FitzGerald we have begun to study how protein synthesis arrest leads to apoptosis and have identified BAK and Mcl-1 as critical players in this process. We have also begun to study how protein phosphorylation regulates sensitivity of cells to immunotoxins by doing knock down studies of tyrosine and serine/threonine kinases and measuring the effect of knock down on immunotoxin mediated cell death. In collaboration with Alan Wayne POB, we have analyzed the response of cells directly isolated from patients with ALL to HA22 killing and are trying to set up an assay that may help predict if patients will respond to treatment. We have also isolated HA22 resistant cell lines and are using these to understand the basis of clinical drug resistance. One of the mechanisms is the loss of ability to synthesize DPH4 due to methylation of the the promoter of that gene.A significant advance is described in Liu, Onda et al in PNAS this year. This paper shows the identification and removal of human B cell epitopes from PE based immunotoxins producing a new immunotoxin predicted to have very low immunogenicity in humans.